Composition The composition of the Uranian atmosphere is different from that of Uranus as a whole, consisting mainly of molecular hydrogen and helium . [16] The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium, was determined from the analysis of Voyager 2 far infrared and radio occultation observations. [17] The currently accepted value is 0.152 ± 0.033 in the upper troposphere, which corresponds to a mass fraction 0.262 ± 0.048. [16] [18] This value is very close to the protosolar helium mass fraction of 0.2741 ± 0.0120, [19] indicating that helium has not settled towards the centre of the planet as it has in the gas giants. [20] The fourth most abundant constituent of the Uranian atmosphere is methane (CH 4 ), the presence of which has been known for some time as a result of the ground-based spectroscopic observations. [16] Methane possesses prominent absorption bands in the visible and near-infrared , making Uranus aquamarine or cyan in colour. [21]

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